173 



When the amount of plant food in a soil is the only point at issue, 

 one needs not to probe to a greater depth than the first foot or eighteen 

 inches, but, it is obvious from what has been stated above, that in order 

 to ascertain not only whether a soil is at present unproductive through 

 brackness, but also whether it is liable to become so when irrigated, it is 

 not enough to take merely a sample of soil from the surface and analyse 

 that. It is essential to take samples at regular intervals below the surface 

 to a depth at least equal to that which the irrigation water may possibly 

 reach. There should be no omission that may leave a loophole for mistake. 

 For this reason the method employed is to take samples representing 

 successive sections of the soil vertically downwards from the surface to a 

 depth of from four to six feet. Generally, one-foot sections are taken, 

 but, in certain cases, ten inch, eight inch, or even three inch sections 



Collecting" Alkaline Soil at Thebus. 



have been collected as occasion seemed to demand. Sometimes it has 

 been advisable to sample solid blocks of soil, for instance, in the Thebus 

 investigation, where the following method was used : 



A hole, several feet in diameter, was excavated to a depth of six 

 feet, a solid pillar of earth being left untouched in the centre. From this 

 central pillar six samples were taken in each case, every sample repre- 

 senting the average of one foot depth of soil. It will be readily seen that 

 such a method of sampling involves more labour than scraping a couple 

 of handfuls of soil from the surface, a method which has sometimes been 

 employed with somewhat misleading results. 



It is to the presence of alkaline salts, comprising the chloride, sul- 

 phate and carbonate of sodium, that the brackness of a soil is due. Of 

 these salts, sodium sulphate is the least and sodium carbonate the most 

 injurious, on account of the corrosive effect of the latter on the bark ; 

 and its puddling of the soil, and so interfering with tillage; the last- 

 named salt is the cause of what is commonly termed black brack, the dark 



